High-porosity porous silicon, after electrochemical oxidation, is a stable and highly reproducible luminescent material with a luminescence quantum efficiency as high as 3% at room temperature. Luminescence decay rates as long as several hundreds of microseconds show that radiative and nonradiative processes both have low efficiencies even at room temperature. This shows that confinement of carriers inside nanometer-sized crystallites does not have a noticeable effect on indirect-band-gap selection rules but restricts strongly the different processes for nonradiative deexcitation. An analysis of the dependence of the nonradiative-decay rates on carrier confinement in terms of the tunneling of carriers through silicon oxide barriers surrounding the confined zone accounts well for our experimental results with an average barrier thickness of 5 nm. This tunneling model is also used to explain successfully the increase in quantum efficiency with the increase of the level of oxidation. © 1992 The American Physical Society.